Quasi 2D Ultrahigh Carrier Density in a Complex Oxide Broken-Gap Heterojunction

Two-dimensional (2D) ultra-high carrier densities are of considerable current research interest for novel plasmonic and high charge-gain devices. However, the highest 2D electron density obtained is thus far limited to 3 × 1014 cm-2 (Electron/unit cell/interface) at GdTiO₃/SrTiO₃ interfaces, and is typically an order of magnitude lower at LaAlO₃/SrTiO₃ interfaces. We show from experiment and modeling that carrier densities much higher than expected based on resolution of the polar discontinuity at perovskite oxide heterojunctions can be achieved via band engineering. The SrTiO₃ (8 u.c.)/NdTiO₃ (t u.c)/SrTiO₃ (8 u.c.)/LSAT(001) heterostructure shows the expected electronic reconstruction behavior starting at t = 2 u.c., but then exhibits a higher carrier density regime at t ≥ 6 u.c. due to additional charge transfer from band alignment. Quasi 2D ultrahigh carrier density is realized at perovskite oxide heterojunctions using broken-gap alignment. The SrTiO₃/NdTiO₃ heterostructure shows the expected electronic reconstruction behavior with electron per unit cell up to a critical thickness (t_critical) of NdTiO₃, but exhibits a higher carrier density regime at t > t_critical due to band alignment and charge transfer.